It is often useful to detect the open/closed state of barriers such as doors and windows. Such information can e.g. be used for alarm systems, etc.
U.S. Pat. No. 6,310,549 B1 presents a wireless security system. Each alarm sensor contains three magnetoresistive sensors and that are capable of detecting the three-dimensional vector of a magnetic field. The sensors detect the orientation of the door or window based upon the earth's magnetic pole. The three-dimensional vector output of the magnetoresistive sensors is received by a microprocessor on-board the alarm sensor. The microprocessor continuously compares the magnetoresistive sensors output with the maximum allowable position of the door or window. In order to calibrate the alarm sensor, prior to using alarm sensor, the initial “closed” position of the door must be programmed using a reset switch. Reset switch is a magnetic reed relay that allows alarm sensor to be initialized whenever alarm sensor is first mounted. To install alarm sensor, alarm sensor should be mounted to the outside of unit, on the door or window that provides entry to the unit. With the door or window in the closed position, the installer will force alarm sensor to reset by using an external magnet. After removing the external magnet from alarm sensor, alarm sensor will calibrate for a closed position reading for a period of time.
US 2014/0001779 A1 presents a system for changing a kicking state of a window and/or a door, comprising at least one handle housing that can be connected in a rotationally fixed manner to the window and/or the door and a handle that is rotatably mounted relative to the handle housing. The system further comprises at least one electronic evaluation circuit for detecting the position of the handle, wherein the evaluation circuit is provided with at least one primed circuit board arranged, at least in part, inside the handle housing and/or the handle, and accommodating a first sensor. It is possible for a calibration mode to be initiated by moving the handle in a predetermined manner back and forth between different positions, particularly locking positions, for example two times between the locking and unlocking position. In doing so, both the 180° movement and the locking events upon engagement in the locking and unlocking position as well as the movements through the partial unlocking position are detected via the sensor. Once this calibration mode has been switched on, the individual locking positions of the handle can then be initiated in a predetermined manner and, at the same time, the different positions of the door or window can be set, particularly a completely open position, a completely closed position and a tilted position and combinations of the positions of the handle and window, with a provision being made in particular that, upon reaching the respective window position, the handle is moved in turn in a predetermined manner between various locking positions in order to inform the system that the respective position of the window or door has been reached.
However, the presented calibration requires the system to be set in a calibration mode. This can be performed on installation, but there is a significant risk that calibration does not occur sufficiently often after that, since the calibration requires specific calibration actions from the user, whereby the signals from the sensor can drift from the calibration.